M4 is a short, transcribed repeat sequence interspersed with singly-copy DNA in the Dictyostelium genome. I have isolated a set of cloned (mRNAs which contain M4 sequences covalently linked to different single-copy sequences. The M4 repeat lies at the 5'-end of these cDNAs. During Dictyostelium development, the expression of the M4 sequences increase greater than 5-fold relative to vegetative growth. Additionally, using single-copy probes to specific M4 mRNAs, I have shown that these individual mRNAs exhibit a relative increase in their developmental expression which is similar to that of the total M4 mRNA population. There may be a co-ordinate increase in expression of the M4 gene set during development. I have additionally, identified an independent transcription-unit whose expression rapidly decreases at the onset of development. Interestingly, this gene lies within 1500 base pairs of a gene with a completely opposite pattern of developmental expression.